One thing is certain about teenagers: Things change, inside and out. That includes teen brains and mental abilities, according to a team of neuroscientists from London. The brain researchers recently reported that a person’s IQ may rise or fall and regions of the brain may change during the teenage years.

An IQ, or intelligence quotient, measures a person’s ability to think and reason. Many researchers had believed it never changes for an individual. If that were true, then your IQ at age...